The passage that best identifies the source of the narrator’s guilt at the end of the story is:
D. "Your grandmother says to tell you that of all the hurtful things you can do to a person, the worst is to make them feel as if they are worth nothing.”
This passage highlights the emotional impact of the narrator's actions on others, particularly how the narrator's behavior may have contributed to feelings of worthlessness in a loved one, which is a central theme of guilt in the story.